What is the purpose of the AT-ACT?

What is the purpose of the AT-ACT? Why need

Look at what the initials stand for: All Terrain, Armored Cargo Transport. The design of the AT-AT and AT-ACT is meant to be:

Heavily armored, so it can withstand a beating. Basically, it’s a huge personnel-carrying tank.

All-terrain. The long legs and quadrupedal walking style gives it the ability to maneuver around almost any kind of terrain. This includes places that would be hard for a ground-based speeder or an air-based shuttle to reach. This includes hilly terrain, steppes, swamps, forests, etc, all without having to prepare a landing area in advance.

(Note that, in the “real world”, engineers are constantly trying to build robots that can walk like 4- or 6- or 8-legged animals rather than roll around on wheels or treads, for the same reason.)